Switching to RNG Improves Local Air Quality
Substituting conventional fossil-fuels such as gasoline and diesel with vehicle fuels derived from renewable natural gas has several environmental benefits, including reducing the emission of atmospheric pollutants, which in turn improves local air quality.
Since renewable natural gas (RNG) combusts in the same manner as fossil-based natural gas, pipeline operators do not distinguish between these two sources after RNG has met the required quality standards and has been injected into the gas pipeline for distribution. While fossil-based natural gas and RNG both consist largely of methane, together with several other common components such as carbon dioxide, nitrogen, oxygen, hydrogen sulfide, sulfur, and trace elements of volatile organic compounds (VOCs), there are slight differences in their composition that ultimately impact air quality. Fossil-based natural gas tends to contain higher levels of these trace VOCs than renewable natural gas, and typically also contains a number of non-methane hydrocarbons such as butane, ethane, pentane and propane that are generally not found in RNG.
Because all new motor vehicles entering the market now have to meet the same emission standards for emissions of carbon monoxide (CO), nitrogen oxide (NOx), and particulates (PM), regardless of the type of fuel used to power them, emissions from new vehicles powered by natural gas are similar to those of new vehicles powered by petroleum-based fossil fuels. But when older vehicles powered by fossil-fuels are replaced with new RNG fueled vehicles, it can result in a decline in levels of local air pollutants arising from vehicle emissions.
Besides the combustion emissions, fossil-fuel powered vehicles also release hydrocarbon emissions when fuel evaporates from vehicle fuel tanks. However, since fuel systems in natural gas powered vehicles are sealed, hydrocarbon emissions arising from evaporation are substantially reduced. Therefore, replacing older fossil-fuel powered vehicles with natural gas powered vehicles, or converting older models to run on natural gas, can result in a reduction in various air pollutants, which improves local air quality.
The Argonne National Laboratory has developed an Alternative Fuel Life-Cycle Environmental and Economic Transportation (AFLEET) tool to assess both the environmental and economic costs and benefits of switching from fossil fuels to RNG by helping fleet owners estimate their petroleum usage, air pollutant and greenhouse gas emissions, as well as comparing the cost of ownership of both light- and heavy-duty vehicles.
An AFLEET analysis of the percentage of pollutant emission reductions attainable if older model gasoline powered pickups (year model 2005-2010) and diesel powered refuse trucks (year model 2006-2012) are replaced by later (2019) model CNG vehicles, illustrates how substantial these emission reductions can be, especially for older models. The analysis showed that replacing older model vehicles (2005 pickups & 2006 trucks respectively) with CNG vehicles would result in the following emission reductions:
- Nitrogen oxide emissions: 87.4% (2005 pickups) & 99.4% (2006 trucks)
- Volatile Organic Compounds: reduction in VOC emissions from combustion - 86% pickups & 93.9% trucks; reduction in VOC emissions due to evaporation - 87.5% pickups & 7.14% trucks
- Particulate Matter: reduction in PM10 - 73% pickups & 97% trucks; reduction in PM2.5 - 68.9% pickups & 96.9% trucks
- Sulfur dioxide: 38.1% pickups & 43% trucks
- Carbon monoxide: 84.3% pickups & -571% trucks
We can see that there is a significant increase in carbon monoxide emissions for CNG refuse trucks compared to those produced by older diesel trucks. The reason for this is that diesel engines have compression ignition cycle engines whereas the newer CNG powered refuse trucks are powered by spark ignition cycle engines with 3-way catalysts. While this results in higher carbon monoxide emissions, these trucks still meet existing emission standards for heavy-duty vehicles. Consequently, when considering replacing older heavy-duty diesel powered vehicles with CNG powered alternatives, it is important that the trade off between reduced nitrogen oxide and particulate matter emissions versus higher carbon monoxide emissions should be taken into account, with regards to both the current local air quality conditions as well as complying with national emission standards.
An Overview of Renewable Natural Gas from Biogas. July 2020, EPA 456-R-20-001.